Field of Art
The present disclosure relates to optical stabilization in scanning imaging system and systems, methods, and non-transitory computer readable medium with instructions for controlling the optical stabilization.
Description of the Related Art
The scanning light ophthalmoscope (SLO) has become an important tool for the study of the human retina in both normal and diseased eyes. For retinal imaging systems such as a SLO, eye movement is a big issue for imaging. The human eye is constantly in motion; even during careful fixation, normal, involuntary, microscopic eye movements, cause the scanned field of the SLO to move continuously across the retina in a constrained pattern. Fixational eye motion causes unique distortions in each SLO frame due to the slow frame rate of the SLO relative to the motion of the eye. In the normal eye, these movements tend to be rather small in amplitude. However, in patients with retinal disease or poor vision, fixational eye movements can be amplified and introduce distortions that are a major hindrance to efficient SLO imaging, in some cases precluding imaging altogether. Unfortunately, these patients are potentially some of the most interesting to study using this technology. It is therefore desirable, particularly for clinical imaging, to minimize or eliminate this motion altogether.
Many state-of-the-art SLOB suffer from two major limitations: small field of view (FOV) and a reliance on patient fixation for targeting a retinal location. Usually an SLO takes multiple images for averaging and constructing panoramic images. For constructing these images, each frame should be at an exact position. This is very difficult because the eye moves continuously during imaging. Especially, in small FOV systems such as an Adaptive Optics SLO (AO-SLO) eye movement can be quite large when compared with the frame size and sometimes the imaging area can go out of frame easily due to the eye movement.
AO-SLO has become widely used to obtain high spatial resolution images from the living human eye. However, normal, involuntary eye movements, even during careful fixation, cause the imaging field to move continuously across the retina in a constrained pattern. Normal fixational eye movements consist of drifts, tremor, and microsaccades. These motions can cause unique distortions in each AO-SLO video frame due to the relatively slow frame rate of AO-SLO. The frame rate of the AO-SLO is set by the scan velocity of the slow scanning mirror and the velocity of the fast scanning mirror.
The velocity of the fast scanning mirror in the AO-SLO is the primary limitation to achieving very high frame rates that would effectively eliminate image distortion within individual frames. The frame rate of the AO-SLOs is limited by the speed of appropriately-sized, fast scanning mirrors, which achieve a maximum frequency of ˜16 kHz. Uses of these fast scanning mirrors require that the AO-SLO image be de-warped (to remove sinusoidal distortions induced from the fast resonant scanner), registered (to facilitate averaging), and averaged (to increase SNR) to generate an image for qualitative or quantitative analysis. Image registration works by recovering the eye motion and nullifying it, and is required to generate high SNR images from AO-SLO image sequences.
In the past, photographic techniques were first used to measure eye movements. One of the earliest attempts to precisely measure the two dimensional motion of the retinal image employed the ‘optical-lever’ method, a technique that measured the light reflected from a plane mirror attached to the eye with a tightly fitting contact lens. The optical lever method was used to both measure eye movements and deliver stabilized stimuli to the retina; this method has achieved very precise optical stabilization, with an error rate of 0.2-0.38 arcminutes, or less than the diameter of a foveal cone (˜0.5 arcminutes). Despite the precision of optical lever method, the invasive nature of this method and its limitations for stimulus delivery limits it usefulness and has been largely replaced with dual-Purkinje image (dPi) eye trackers. The dPi eye trackers use the Purkinje images (i.e. images of a light source reflected from the cornea and lens) to non-invasively measure eye position. The dPi eye trackers measure eye motion and manipulate visual stimuli with a precision of ˜1 arcminute. Each of these methods indirectly infer the motion of the retinal image.
One solution to the problem of imaging live retinas is to use post-processing which can cause a long delay in delivering the video or require large amounts of processing power. Most clinical and experimental uses of the instrument require that the raw AO-SLO image be de-warped and stabilized to present the user with an interpretable image. Another solution to this problem is an eye position tracking system. Prior art eye position tracking systems used mostly open loop control algorithms. These eye tracking systems detect eye position using a position detection apparatus and shift the imaging area according to the eye movement using tracking mirrors. Image based position calculation methods including methods which detect specific features can be used in the position detection apparatus. In addition, two scanning mirrors are used as tracking mirrors.
Implementing an eye position tracking system raises the cost of the overall system. Two galvano scanners are needed as tracking mirrors in addition to the resonant scanner and the galvano scanner. In addition, each tracking mirror should be conjugate with the pupil or the eye rotation center. These additions add to the complexity of the optical system and increase costs. The following disclosure is directed towards providing a better solution to this problem. The following disclosure is also directed towards implementing this solution into a system that includes both a narrow FOV SLO and a wide FOV SLO.